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How-to: Perform an Overlay Test

 
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Stratadrake
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Joined: 05 May 2004
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PostPosted: Sat Nov 03, 2007 11:18 am    Post subject: How-to: Perform an Overlay Test Reply with quote

If you run across a submission that looks like it has been traced from another work, how do you know for sure?

The answer's obvious: You can locate the original piece, overlay them together in Photoshop, and --

But hold on there. The hard part about traced works is that the two images may not match up in size. Sure, it's pretty easy to perform an overlay test when they do -- in fact, in these cases you can simply open both images up in your browser tabs and then switch back-and-forth between them -- but when you ask Photoshop (or any raster editor, for that matter) to resize an image, you have to input some kind of scalar or destination size. What about when you don't know the scalar or size you need? Then it becomes a game of trial-and-error, repeatedly resizing the image and hitting the "Undo" button before trying again. Not to mention that resizing/rotating an image in raster changes the image itself, and can potentially take a long time with large images. Isn't there an easier way to overlay and adjust images until it "looks about right"?

Yes there is -- and enter the vector graphics editors.

If you have Adobe Illustrator, you'll probably know how to do this already, but if you don't, here are some following steps to show you how to perform an overlay test.

1 - First, download and install Inkscape (http://www.inkscape.org/). Inkscape is an open-source vector graphics editor available for most operating systems, meaning it is both powerful, free, and has no strings attached.

2 - In order to perform an overlay test you will need to download both images -- the original image and the "traced" image, on to your hard drive. Download them somewhere convenient where you can find them easily.

3 - If this is your first time running Inkscape, then immediately after starting it up go to the Help menu and select Tutorials, Inkscape: Basic. Seriously. Don't continue until you know how to operate Inkscape itself.

4 - Once you're ready to perform an overlay test, go to the File menu, select Import..., and load the original image (from step 2). This will add it to the image as a raster object. You can't modify the object directly (that's what raster editors, à la GIMP and Photoshop, are for!), but you can freely move, scale, and rotate the object in its entirety by click-and-drag, and the results are visible immediately.

5 - Next, pick the Import... command again and load the traced image (also from step 2). This will add it to the document too.

6 - One of the two objects will overlap on "top" of the other one, obscuring it. Click the top object to select it, and then locate the opacity setting on the status bar at the bottom (it should read "100", and when you hover the mouse pointer over it a popup will appear saying "Master opacity, %").

7 - Click on the Opacity control and adjust to approximately fifty percent. This will make the top (traced image) object partially transparent, allowing you to see the bottom (original image) object through it.

CAUTION: Inkscape versions through 0.47 have a rendering bug which causes the opacity of imported bitmaps to be squared, so in these versions you will need an opacity of "70%" to get fifty percent transparency. (This bug will be fixed in Inkscape 0.48 ).

8 - NOW you are ready to perform the overlay test. This is the easiest part of all: Simply click and drag the top object on top of the bottom object, and then click and drag on its scaling handles to resize the object. The results are visible immediately; whereas Photoshop would ask you for a scalar, Inkscape allows you to scale the object until it "looks right".

9 - The verdict? If significant details (lineart such as outlines of a character's face or body) are a perfect match with the original piece, then chances are good you're looking at a trace or "draw-over" of the original work.
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Last edited by Stratadrake on Tue Aug 24, 2010 9:04 am; edited 2 times in total
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Stratadrake
Grammar nazi
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Joined: 05 May 2004
Posts: 13689
Location: Moo

PostPosted: Sat Dec 27, 2008 9:52 am    Post subject: Reply with quote

I am going to add two important points with regards to the above steps:

  1. Bug Alert!
    There is currently a glitch in Inkscape's rendering of raster objects, causing their opacity value to be applied twice. As a result, if you specify an Opacity value of 50% then the object will actually be rendered at 25% opacity. So in order to render at 50% opacity you will need to specify an opacity value of 70%.

  2. Reporting to Site Staff
    Please avoid using the terms "pass" or "fail" in the context of a tracing test. Simply say whether or not you believe it constitutes a trace. Sometimes we cannot easily call it one way or the other, and it is safer for FAC staff to take no action if the results of a tracing test are ambiguous.

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Stratadrake
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Joined: 05 May 2004
Posts: 13689
Location: Moo

PostPosted: Thu Jan 01, 2009 8:56 am    Post subject: Reply with quote

Another quick update:

  • Pseudo Overlay Test
    Sometimes a traced picture will have the exact same pixel dimensions (width and height) as the original picture. In these cases you actually don't need GIMP or Inkscape to perform a tracing test at all. Just open both images in adjacent browser tabs and switch back-and-forth between them. If one was traced from the other then the match will be obvious.

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